A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Although lithographic apparatus are operated in clean rooms and flushed with clean air, contamination of the apparatus does occur and, depending on the location and type of contaminant, causes various problems. For example, inorganic contaminants on the mask deriving from the air in the clean room or from manufacture, transportation and storage of the mask can cause localized absorption of the projection beam leading to dose errors and improper imaging of mask features or even printing of marks in what should be blank areas. Particulates on the substrate table can distort the substrate leading to localized focus errors (known as hot spots). When due to contamination on a substrate table, the substrate adopts a bended position, and the pattern on the surface of the bended substrate does not correspond to the pattern intended to be transferred to the substrate, thereby contributing to bad overlay.
In addition to the ambient air and the manufacture, etc., of masks and substrates, sources of contamination include resist debris sputtered from the substrate by the projection beam during exposures, and mechanical contact between moving parts of the apparatus, which may cause particulates to be dislodged from the contacting surfaces. Contamination may also include metal and/or oxide particles.
Most of the contamination is presumably entering the lithographic apparatus via the substrates which have been treated in processes prior to entering the apparatus. In particular, resist contamination is thought to enter the apparatus via the wafers.
To minimize errors caused by contamination, susceptible parts of the apparatus, such as masks, mask tables, and substrate tables, are cleaned frequently. This generally is a time-consuming manual task, taking two hours or more to clean a substrate table, for example, which causes undesirable downtime of the apparatus and must be carried out by skilled engineers. On occasion, manual cleaning fails to remove the contaminants and must be repeated. Selective cleaning of a burl table is disclosed in EP-1 093 022-A in which an abrasive tool or electromagnetic radiation of unspecified form is used. U.S. Pat. No. 6,249,932 discloses a manual cleaning head that uses blown air and vacuum for cleaning a table in a lithographic projection apparatus. Various methods of cleaning substrates are known—see e.g. WO 02/053300 and WO 02/42013—but these require the substrates to be placed in special machines.
To overcome the problems of downtime, the lithographic apparatus may be provided with a cleaning device for cleaning in situ a component in the lithographic apparatus, as disclosed in EP 1329773 A2. The cleaning device may provide a laser to ablade and/or thermally dislodge contaminants. The wavelength of the laser beam is chosen such that absorption by the contaminants that are expected to be present on the component be cleaned, is maximal. When a short pulse length is used, in the order of less than 100 nanoseconds, the sudden thermal expansion difference between the heated contamination and the component may cause a shockwave, resulting in G-forces high enough to loosen the contamination from the component. A clean device may additionally, or alternatively, provide in a non-ionizing, low pressure environment around the component to be cleaned, so that electrostatic forces due to a potential difference between a cleaning tool and the component to be cleaned are created. The potential difference and separation between the tool and the component to be cleaned that are needed to remove contaminants depends on the contaminants to be removed and the properties of the surface to which they are adhered. In other words, in either way, successfully operating the cleaning device depends on prior knowledge about the type of contamination to be removed.